Rotor stability of a rotary pump

ABSTRACT

A rotary pump including an impeller rotatable within a housing. A load is imposed on the impeller as it rotates, in a direction that is substantially parallel to the axis of rotation and wherein the load stabilises the motion of the impeller. The load may be achieved by magnetically biasing the impeller.

TECHNICAL FIELD

The present invention relates to improvements in the impeller or rotorstability of a rotary pump.

BACKGROUND OF THE INVENTION

Rotary pumps have used journal bearings to stabilise a spinning rotor orimpeller. It has been noted that commonly the impeller rotates about acentral axis within the pump housing and may whirl to the side of thehousing when the impeller is rotated at high speeds with insufficientload and results in instability in relation to the rotation of theimpeller.

The impeller of the rotary blood pump disclosed within U.S. Pat. No.6,227,797—Watterson et al. is hydrodynamically suspended and may undercertain conditions the impeller or rotor may experience a touchdownevent. A touchdown event is a situation where the impeller or rotortouches or contacts the inner walls of the pump housing. Touchdown ofthe impeller or rotor often leads to damaging the impeller, housingand/or the pumping fluid. If a touchdown event occurs in a rotary bloodpump implanted in a patient, such an event may result in impaired pumpperformance that may result in complications for the patient. Touchdownmay be avoided by increasing the stability of the impeller or rotor orincreasing the stiffness and/or dampening of the impeller or rotor.

U.S. Pat. No. 5,324,177—Golding et al describes a means for increasingimpeller or rotor stability in a rotary pump. The impeller and/or rotorare biased by a load provided by additional load acting only in radialorientation in respect of the axis of rotation of the pump. This has theeffect of offsetting the rotor and thus stabilising the rotor in onlythe radial direction. The arrangement disclosed in U.S. Pat. No.5,324,177, may also tend to destabilise the impeller in relation to theaxial positioning of the impeller. Additionally, the radial biasing ofthe impeller is only useful in situations where the motor stators of thepump are positioned radially in respect of the axis of rotation of theimpeller. This may lead to a considerably increase in size of theoverall pump.

The present invention aims to at least address or ameliorate one or moreof the above disadvantages associated with the abovementioned prior art.

SUMMARY OF THE INVENTION

In accordance with a first aspect the present invention consists in arotary pump including an impeller rotatable within a housing; wherein aload is imposed on said impeller as it rotates, in a direction that issubstantially parallel to the axis of rotation and wherein said loadstabilises the motion of the impeller.

Preferably said load is achieved by magnetically biasing said impeller.

Preferably said pump includes a set of stators positioned below theimpeller and said set of stators generates said load.

Preferably the magnetically biasing of said impeller is achieved by atleast one yoke.

Preferably an angle formed between the upper and/or lower axial surfacesof the impeller are not parallel with the respective correspondingsurface of the inner wall of the housing.

Preferably said impeller is not circular and the inner walls of thehousing are generally circular.

Preferably said load prevents or limits the impeller from contacting thehousing, when in use.

Preferably said impeller is generally square shaped.

Preferably said impeller includes a hydrodynamic bearing.

Preferably said hydrodynamic bearing is formed by a taper on the surfaceof the impeller of between 10 μm and 50 μm.

Preferably a gap of less than 250 μm is formed between the impeller andthe housing, when in use.

Preferably said pump is for pumping blood.

Preferably said pump is implantable within the body of a patient.

In accordance with a second aspect the present invention consists in arotary pump including an impeller rotatable within a housing; whereinimpeller is hydrodynamically suspended and wherein an angle formedbetween the upper and/or lower axial surfaces of the impeller are notparallel with the respective corresponding surface of the inner wall ofthe housing.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described withreference to the accompanying drawing wherein:

FIG. 1 shows a cross-sectional top view of an example of the prior artin this field;

FIG. 2 shows a cross-sectional side view of a first embodiment of thepresent invention;

FIG. 3 shows a perspective view of a portion part of the embodimentshown in FIG. 2; and

FIG. 4 shows a cross-sectional side view of a second embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 schematically depicts a prior art rotary pump 3 having agenerally circular impeller 1 positioned within a pump housing 2 with agenerally circular inner wall. The impeller 1 is preferably suspended bya fluid or hydrodynamic bearing generated by the interaction of theouter surface of impeller 1 and the inner surface of pump housing 2.

The impeller 1 rotates about a central axis of the rotary pump 3. InFIG. 1, the impeller 1 is shown to be under an insufficient biasingload. This insufficient load biasing the impeller allows it to move awayfrom the central axis of rotation in a helical pattern 4. This resultsin the impeller 1 experiencing a relative instability which in the worstcase may lead to impeller instability. The impeller 1 may in this caseroll around the inner surface of the housing 2 and follow the helicalpath 4, which may lead to the eventual collision with the inner wall ofthe housing 2. This motion may result in damage to the outer surfaces ofthe impeller 1 against the inner walls of the housing 2 and/or damagingthe pumping fluid.

A first embodiment of the present invention is shown in FIG. 2. In thisembodiment, there is provided a rotary pump 3 including an impeller 1which is hydrodynamically suspended within housing 2 using a fluidbearing. When in use, the impeller 1 rotates about the axis of rotationand pumps fluid from the inlet 7 to the outlet 6 by a continuouscentrifugal motion. The pumping fluid is preferably blood and the rotarypump 3 is suitable for implantation within the body a patient.

The fluid bearing is achieved by the interaction of the outer surface ofthe impeller 1 interacting with the inner surface of the housing 2 ofthe rotary pump 3. The rotary pump 3 is preferably adapted to beimplantable within the body of a patient to assist the pumping fluid,such as blood. The rotary pump 3 includes: stators 9, an inlet 7, and anoutlet 6. The stators 9 are preferably mounted axially, in relation tothe axis of rotation of the rotary pump 3, on or in the housing 2 andimpart an electrodynamic driving force on magnets encapsulated withineach of the blades 10 which form the impeller 1. The impeller 1comprises four blades 10 that are connected by struts 11 in a generallysquare configuration and the blades 10 also include hydrodynamic bearingsurfaces which form fluid bearings when they interact with the innersurface of the housing.

When in use, a load may be created electromagnetically by the stators 9to act on the impeller 1 in a direction that is substantially parallelto the axis of rotation A. This load acts on the impeller 1 in an axialdirection and biases the impeller 1 either generally towards the inlet 7or towards the lower inner surface of the housing 2. The axial biasedload acting on the impeller 1 may additionally stabilise the rotatingimpeller 1 and may improve the stability of impeller 1.

The electromagnetic biasing may be achieved by either: increasing theEMF output of the stators on either the upper or lower side of theimpeller 1; or by introducing a yoke which contacts the stators andincreases the EMF output on either the upper or lower side of theimpeller 1.

Alternately, the load may be created by inducing an axial magnetic load.This magnetic load may be formed or created by including a ring of ironor Permalloy™ material above and/or below the stators 9 in the housing.The ring may form a yoke 15 covering the stators 9. Preferably, the ringon either side of the impeller 1 must be of varying amounts of iron orbe of varying distances from the impeller 1. The effect of which wouldbe to vary the load experienced by the impeller 1 as the magnetsencapsulated with the impeller 1 may be drawn to a yoke 15.

Preferably, the gap 8 is formed between the outer axial surface of theimpeller 1 and the corresponding inner wall of the housing 2. The gap 8is preferable optimised when the gap 8 is less than 250 μm, as this mayincrease stiffness and dampening of the bearing and lead to increases inrotor stability.

Also preferably, the blades 10 of the impeller 1 include a taperedsurface. The tapered surface is preferably optimised at a height ofbetween 10 μm and 50 μm. This also may allow the impeller or rotor to beadditionally stabilised.

FIG. 3 shows a preferred impeller 1 which may be used with the firstembodiment of the present invention. This impeller 1 includes fourblades 10 joined by four struts 11. The blades 10 may includehydrodynamic surfaces to allow the impeller 1 to be hydrodynamicallysuspended, when in use. The blades 10 preferably have a generally ‘sharkfin’ shaped configuration so as to minimise damage to the pumping fluid.The impeller 1 has a generally square configuration, wherein the struts11 are joined between the outer adjacent edges of the blades 10. Thestruts 11 may also include hydrodynamic surfaces. The preferred impeller1 is shaftless to also minimise damage to the pumping fluid.

The impeller 10 shown in FIG. 3, may also experience an additional load,when in use. This load may be generated by the generally squareconfiguration of the impeller 10 interacting with a generally circularinterior surface of the pump housing 2. The non-uniform shape of theimpeller 10, as depicted FIG. 3, may allow the forces acting on theimpeller to be decentralised and this may increase the load acting onthe impeller 10. The load thereby limits the instability experienced bythe impeller 10. A further embodiment of the present invention is shownin FIG. 4. This embodiment shows an alternate impeller 1, wherein theupper 13 and lower 12 axial outer surfaces of the impeller 1 have beenpositioned to be not parallel with the respective corresponding surfaceof the inner of the housing 2. The effect of this feature is to increasethe stiffness and dampening of the bearing, when in use. This in turnleads to an increase in relation to the stability of the rotor orimpeller 1 and may prevent or limit touchdown.

The embodiment shown in FIG. 4 could be achieved by using the embodimentshown in FIG. 2 and rotating the blades 10 inwards towards the centre ofthe impeller 1. The inward angle depends on the position of the centerof gravity relative to the center of pressure on the bearing surface ofthe impeller 1.

A person skilled in the art will recognise that the term impeller withinthis specification has substantially the same meaning as rotor. All ofthe preferred embodiments may be used as implantable medical devices oras cardiac assist devices.

The above descriptions describe only some of the embodiments of thepresent invention. Further modifications may be obvious to those skilledin art and may be made without departing from the scope and spirit ofthe present invention.

1. A rotary pump including an impeller rotatable within a housing;wherein a load is imposed on said impeller as it rotates, in a directionthat is substantially parallel to the axis of rotation and wherein saidload stabilises the motion of the impeller.
 2. The rotary pump asclaimed in claim 1, wherein said load is achieved by magneticallybiasing said impeller.
 3. The rotary pump as claimed in claim 2, whereinsaid pump includes a set of stators positioned below the impeller andsaid set of stators generates said load.
 4. The rotary pump as claimedin claim 2, wherein magnetically biasing said impeller is achieved by atleast one yoke.
 5. The rotary pump as claimed in claim 1, wherein anangle formed between the upper and/or lower axial surfaces of theimpeller are not parallel with the respective corresponding surface ofthe inner wall of the housing.
 6. The rotary pump as claimed in claim 1,wherein said impeller is not circular and the inner walls of the housingare generally circular.
 7. The rotary pump as claimed in claim 1,wherein said load prevents or limits the impeller from contacting thehousing, when in use.
 8. The rotary pump as claimed in claim 6, whereinsaid impeller is generally square shaped.
 9. The rotary pump as claimedin claim 1, wherein said impeller includes a hydrodynamic bearing. 10.The rotary pump as claimed in claim 8, wherein said hydrodynamic bearingis formed by a taper on the surface of the impeller of between 10 μm and50 μm.
 11. The rotary pump as claimed in claim 1, wherein a gap of lessthan 250 μm is formed between the impeller and the housing, when in use.12. A rotary pump as claimed in claim 1, wherein said pump is forpumping blood.
 13. A rotary pump as claimed in claim 13, wherein saidpump is implantable within the body of a patient.
 14. A rotary pumpincluding an impeller rotatable within a housing; wherein impeller ishydrodynamically suspended and wherein an angle formed between the upperand/or lower axial surfaces of the impeller are not parallel with therespective corresponding surface of the inner wall of the housing.